This invention relates to the field of amplitude modulated stereo broadcasting and, more particularly, to a signal providing for minimal envelope distortion with minimal adjacent channel interference.
The simplest known signal for providing AM stereo broadcasting is probably the well known pure quadrature signal. Since the amplitude of a pure quadrature signal is the square root of the sum of the squares of the sum and difference signals, rather than the sum signal as in AM monophonic broadcasting, the quadrature signal is not compatible; i.e., a pure quadrature signal would produce a severely distorted audio output in a monophonic envelope detector.
A system providing no distortion in monophonic receivers was disclosed in a U.S. Pat. No. 4,218,586, assigned to the same assignee as is the present invention, in which the amplitude is always 1+L+R, and the instantaneous phase angle .phi. is the cosine of arc tan [(L-R)/(1+L+R)]. While this signal is ideal for all normal program material, under extreme program conditions such as a high frequency signal in one channel only with a high modulation level, a slight increase in the amount of adjacent channel interference is possible. The only known systems with no spurious adjacent channel interference-producing sidebands are quadrature systems, and attempts have been made to provide a practical quadrature signal by reducing the level of the difference signal. Various percentages of reduction have been tested, but any slight improvement in distortion in monophonic receivers has been outweighed by the loss in signal-to-noise ratio in the stereophonic receivers due to the necessity of increased gain to compensate for the reduced difference signal.